1. Field of the Invention
This invention relates generally to a biodiesel production method for producing and washing biodiesel fuel.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
It is known for a biodiesel fuel, i.e., lower alkyl esters of higher fatty acids, to be produced from an oil phase and lower alcohols in a catalytic transesterification process in the presence of an alkaline catalyst. For example, U.S. Pat. No. 5,354,878 issued 11 Oct. 1994 to Connemann et al. discloses a two-stage process for producing biodiesel as well as a process for washing biodiesel produced by the two-stage process. In a first transesterification and glycerin separation stage the Connemann et al. disclose mixing raw oil comprising oil-phase fatty acids, with a solution of methanol (MEOH) and an alkaline catalyst comprising sodium methylate at 150% stoichiometric MEOH. The resulting first stage reaction mixture is pumped into a first reactor. A glycerin phase is separated from the resulting first stage reaction product stream by passing the first stage reaction product through a first centrifugal separator. The glycerin phase of the first stage reaction product leaves the centrifugal separator in one stream and a biodiesel phase of the first stage reaction product is discharged in a second stream.
In the second stage, the above steps are repeated by mixing a non-recycled portion of the biodiesel phase of the first stage reaction product stream with additional alkaline catalyst. The resulting second stage reaction mixture is passed into a second reactor. Glycerin is separated from the resulting second stage reaction product stream by passing it through a second centrifugal separator. A resulting biodiesel phase of the second stage reaction product is passed (at a reaction state of 99.2 to 99.6% complete) to a stripping stage.
In the stripping stage, surplus unreacted methanol is stripped from the biodiesel phase of the second stage reaction product. The unreacted methanol is stripped by passing the biodiesel phase of the second stage reaction product through a vacuum distillation tower. The resulting evaporated methanol passes through a condensation system to be condensed and passed back to be recycled for use in the first stage of the Connemann et al. process. The stripped biodiesel phase of the second stage reaction product is passed to the next process stage.
From the stripping stage the stripped biodiesel phase of the second stage reaction product (or unwashed biodiesel product) passes through a first water wash step in which catalyst residue, water, soap, and remaining glycerin are washed from the unwashed biodiesel product by passing the unwashed biodiesel product along with water through a third centrifugal separator and operating the centrifugal separator to separate catalyst, soap, and glycerin from the unwashed biodiesel product. The resulting washed biodiesel product may optionally be passed through a second water wash step in which the washed biodiesel product is passed, with water, through a fourth centrifugal separator to remove additional NaOH, glycerin, and soap, resulting in a final biodiesel product having a purity of 99.2-99.6%.
The Connemann et al. patent therefore discloses a slow, unpressurized, low-temperature process that includes the use of reactor columns and at least two glycerin extraction points and a recycle circuit that recycles a portion of the biodiesel phase of the reaction product of each transesterification stage. More specifically, the Connemann et al. patent discloses the use of a pump mixer to both mix and propel the reaction mixture into the top of a reactor tank column, and beginning the transesterification reaction by passing the reaction mixture downward through the reactor tank column at a flow rate limited to a rate lower than a sinking rate of the glycerin separated from the reaction mixture. The downward flow of the reaction mixture through the reactor tank column must be slower than the glycerin sinking rate to allow sufficient time for transesterification. Consequently, to drive the reaction and remove sufficient glycerin, the Connemann et al. process includes the additional steps, in each transesterification stage, of pumping reaction mixture into a glycerin settling tank before reaching the centrifugal separator and then recycling a portion of the biodiesel phase of the reaction product stream through the glycerin settling tank. Both the reaction mixture and the recycled portion of the biodiesel phase of the reaction product stream experience a settling tank residence period of from 2 minutes to 2 hours (preferably 1 hour). Because a portion of the biodiesel phase of the reaction product stream is continuously recycled, only a portion of the biodiesel phase of each stage's reaction product stream is forwarded to subsequent stages in the process.
It's also known in the art to use a POD counterflow centrifugal separator in counter-current extraction processes. The POD is manufactured by and available from B&P Process Systems of Saginaw Mich. and includes a first inlet for receiving a first liquid into a radially outer region of a centrifuge tank of the POD. The centrifuge tank is rotatably supported on a centrifuge axis. The POD also includes a second inlet for receiving a second liquid into a central axial region of the centrifuge tank, the second liquid having a specific gravity greater than that of the first liquid. In a counter-current extraction process the centrifuge tank of the POD is spun about the centrifuge axis such that the second liquid is forced radially outward from the central axial region through the reaction mixture and through a series of concentric perforated cylindrical panels supported coaxially within the centrifuge tank in the path of the second liquid. This forces the second liquid to pass through the perforations in the panels, presenting additional surface area contact between the first and second liquids to allow one or more components of the second liquid to dissolve into or otherwise react with the first liquid. The POD further includes a first outlet that discharges the first liquid less the reacted or dissolved component and a second outlet that discharges the second liquid plus the reacted or dissolved component from the first liquid.
It would be desirable if biodiesel could be produced more quickly and using less equipment but without sacrificing purity.